Optical grade (Gd <sub>0.95- <i>x</i> </sub>Lu <sub> <i>x</i> </sub>Eu <sub>0.05</sub>) <sub>3</sub>Al <sub>5</sub>O <sub>12</sub> ceramics with near-zero optical loss: Effects of Lu <sup>3+</sup> doping on structural feature, microstructure evolution, and far-red luminescence
Zhigang Sun, Ji‐Guang Li, Huiyu Qian, Yoshio Sakka, T. Suzuki, Bin Lü
Abstract
A chemical co-precipitation strategy was employed to synthesize a series of (Gd<sub>0.95-<em>x</em></sub>Lu<em><sub>x</sub></em>Eu<sub>0.05</sub>)<sub>3</sub>Al<sub>5</sub>O<sub>12</sub> (<em>x</em> = 0.1−0.95) powder phosphors, followed by vacuum sintering to achieve transparent garnet ceramic phosphors. The density functional theory indicated Lu<sub>3</sub>Al<sub>5</sub>O<sub>12</sub> was formed in priority compared with Gd<sub>3</sub>Al<sub>5</sub>O<sub>12</sub> during solid-phase reaction. Upon high-temperature sintering, the Lu<sup>3+</sup> substitution for Gd<sup>3+</sup> suppressed point mass diffusion leading to a smaller grain size. The in-line transmittances of the bulk specimens with <em>x</em> = 0.1, 0.3, 0.5, 0.7, and 0.95 nm were ~83.5, 80.1, 68.8, 73.7, and 82.2% at 710 nm (Eu<sup>3+</sup> emission center), respectively, among which the <em>x</em> = 0.1 sample exhibited optical grade with near-zero optical loss in agreement with the defect-free single crystal (~100% of the theoretical transmittance). The resulting particle and ceramic materials both presented characteristic Eu<sup>3+</sup> emission arising from <sup>5</sup>D<sub>0</sub>→<sup>7</sup>F<em><sub>J</sub></em> (<em>J</em> = 1−4) transition, where the dominant far-red emission at ~710 nm arising from <sup>5</sup>D<sub>0</sub>→<sup>7</sup>F<sub>4</sub> transition overlapped with the absorption of phytochrome <em>P<sub>FR</sub></em>. The photoluminescence excitation and photoluminescence intensities of (Gd<sub>0.95-<em>x</em></sub>Lu<em><sub>x</sub></em>Eu<sub>0.05</sub>)<sub>3</sub>Al<sub>5</sub>O<sub>12</sub> powders and ceramics generally increased at a higher Gd<sup>3+</sup>/Lu<sup>3+</sup> ratio. Lu<sup>3+</sup> dopants delayed the fluorescence lifetimes while the bulk samples had shorter lifetimes than the particle counterparts. The transparent (Gd<sub>0.85</sub>Lu<sub>0.1</sub>Eu<sub>0.05</sub>)<sub>3</sub>Al<sub>5</sub>O<sub>12</sub> ceramic phosphor exhibited good thermal stability with a high thermal quenching temperature above 533 K. The designed ceramic phosphor converted light-emitting diode had a saturation injection current of 435 mA and current-dependent color rendering index. More importantly, our report marked the developmental stage of transparent ceramic materials towards zero optical loss.